The present invention relates to boreholes and particularly to boreholes that are drilled in the search for petroleum deposits.
In drilling boreholes in the search for petroleum deposits, particularly in the Gulf Coast area, both onshore and offshore, the borehole often penetrates shallow water-bearing sand formations. These water-bearing sand formations range in depth from 1000 to a few thousand feet below the surface or, in the case of an offshore well, below the mudline. When the surface casing is subsequently installed in the well and cemented in place, the water in these sands may flow from behind the surface casing washing out the sand grains and/or cement, producing voids. The washed out cement and voids reduce the structural integrity of the casing installation and the cementing of the casing in the formation is seriously impaired. The reduction in the structural integrity results in the inability of the casing to contain the water flow and to support the wellhead equipment that is installed on the top of the casing and is required for drilling of the borehole. This is a particular problem in offshore wells where the surface casing must not only support the wellhead equipment but also a portion of the weight of the conductor pipe that is suspended from the drill rig.
In the past various attempts have been made to isolate these shallow water sand formations to prevent their subsequently damaging the structural integrity of the surface casing and its cementing to the borehole. These attempts have usually comprised the use of drilling muds in an attempt to deposit a filter cake over the sand formation to partially consolidate the formation adjacent the borehole, shut off the water flow, and hold the soft formation in place by hydrostatic pressure until the casing is cemented in the drilled hole. These attempts have had only moderate success because the filter cake itself never hardens and the Portland cement normally used is not compatible with, and will not bond to, the mud cake. Small channels are left through which the produced water will flow, washing out the mud cake and the soft formation, destroying the support for the casing system.
An additional problem arises very frequently in the offshore well cases where seawater, rather than mud, is used to circulate the drill bit cuttings from the borehole while drilling the well, prior to setting the surface casing. The seawater is not sufficiently dense to hydraulically support the soft formation borehole walls, a competent filter cake is not deposited to help consolidate the formation, and water flows from exposed water. bearing zones causing the incompetent formation to crumble and slough even more. A poor cement job is then obtained and subsequent water flow further undermines the casing.
Attempts have been made to improve the cement bond by using mechanical means for removing the mudcake from the borehole wall and displacing sea water from the annulus between the casing and the borehole prior to cementing. These attempts have not been successful because removal of the mudcake allows the water to flow from the sand and destroy the structural integrity of the bond between the cement and the borehole wall. Furthermore, mechanical methods of filter cake removal lead to damage to the exposed borehole face, leading to further crumbling.
Grouting with fine ground slag cement is known. Such as disclosed in Clarke, U.S. Pat. No. 4,761,183 (Aug. 2, 1988) and Clarke, U.S. Pat. No. 5,106,423 (Apr. 21, 1992). The '423 patent also discloses petroleum as an area of utility for the cementitious slurries described in the invention.